Molybdenum complexes bisalkynes

Several mixed bisalkyne molybdenum complexes have been prepared [Eq. (32)] (87). Alkyne exchange is known to occur in the Mo(CO)-(RC=CR)(S2CNMe2)2 reagents (90), and this complicates isolation of pure mixed bisalkyne products. 

The only report of tungsten bisalkyne bisdithiocarbamate complexes to date is a footnote which describes the formation of W(PhC=CPh)2-(S2CNEt2)2 from W(CO)(PhC=CPh)(S2CNEt2)2 and PhC=CPh in boiling methanol (91). The abundance of bisalkyne molybdenum complexes in... 

Few monomeric d4 bisalkyne complexes with only monodentate ligands in the coordination sphere have been reported. The only molybdenum(II) complex in this category is Mo(PhC=CR)2(CO)(NCMe)I2 (R = Ph, Me), which was included in a reaction scheme illustrating products accessible from cleavage of the iodide bridges in dimeric [Mo(PhC=CR)(/r-I)-(I)(CO)(CNMe)]2 reagents (51). Efforts to convert Mo(RC=CR)(CO)-L2X2 complexes to bisalkyne derivatives were not successful (46). 

Molybdenum bisalkyne products form directly when Mo(CO)2-(S2CNEt2)2 and excess alkyne are heated in methylene chloride for 18 hours (52), or, more quickly, in refluxing benzene or toluene [Eq. (31)] (87). The parent acetylene complex, Mo(HC=CH)2(S2CNMe2)2, has... 

Molybdenum bisalkyne complexes form more readily in the pyrrole-/V-carbodithioate ligand system ( pyrroledithiocarbamate ) than in the corresponding dialkyldithiocarbamate systems (88). The pyrrole nitrogen is reluctant to share electron density with the attached CS2 moiety since the aromatic stabilization of the five-membered NC4 ring is lost in resonance form ii. As a result of decreased electron donation from the... 

Bisalkyne cyclopentadienyl derivatives of molybdenum and tungsten have been prominent in the development of Group VI alkyne chemistry in the 1980s. Unlike the dithiocarbamate systems where addition of a second alkyne produces a substitutionally inert bisalkyne derivative, interconversion of monoalkyne and bisalkyne cyclopentadienyl complexes is facile and synthetically useful. 

Another route to neutral bisalkyne complexes is from the trifluoro-methylacyl precursor which deinserts carbon monoxide to yield trifluoro-methyl molybdenum products. Photolysis of CpMo(CO)3[C(0)CF3] in the presence of CF3C=CCF3 forms CpMo(CF3C=CCF3)2(CF3) CpMo-(DMAC)2CF3 is formed without photolysis (98). Addition of hexafluoro-butyne to CpMo(CO)(MeC=CMe)(CF3) forms the mixed bisalkyne via CO substitution. 

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